The present disclosure relates to an image reading device for reading an original document, an image forming apparatus including the image reading device, and a non-transitory computer-readable recording medium storing a program for reading an original document.
An image reading device includes a light source and an imaging section. Typically, the image reading device reads an original document by detecting, with the imaging section, light that is emitted from the light source and then reflected from the original document. More specifically, the image reading device reads an image formed on an original document to generate image data. Such an image reading device is usable as a scanner. In addition, such an image reading device is also usable as a component of an image forming apparatus, such as a multifunction peripheral, a copier, or a facsimile.
Even when one image reading device reads the same original document, the amount of light detected by the imaging section may fluctuate for each reading operation. For example, the amount of light detected by the imaging section may fluctuate depending on changes in the ambient conditions. The amount of light detected by the imaging section may also fluctuate depending on changes in the light source, the optical system, and/or the imaging section. For this reason, the image reading device typically performs shading correction with the use of a density reference member to reduce the fluctuations in the reading result caused by the fluctuations in the amount of light.
By reading the density reference member, the density values of the pixels (the smallest elements of electronically coded image) are detected. The pixels constitute image data representing the density reference member. Ideally, the density values are generally uniform. Otherwise, it is ideal that the change among the density values be gradual. However, the density value of one pixel may be lower than the density values of other pixels. For example, if the light source has a defective portion, the density value of each pixel corresponding to the defective portion is lower than the density values of other pixels. If the density value of any pixel is lower than the density values of other pixels, the grayscale is set to fall within a narrow density range. Consequently, the quality of read images (the reading result) degrades.
Some image reading devices detect a defect in the LEDs based on the reading result of a white plate. More specifically, the level of the amount of light obtained by reading the white plate is compared with a threshold stored in advance to detect a defect in the LEDs. Any LED found to be defective is not used in reading and thus the size of a readable original document is limited.